1. Field of the Invention
The present invention relates to the art of detecting faults and other characteristics in materials, and more particularly to methods and apparatus for detecting faults and other characteristics in ultrasonically vibrated test material using homodyne interferometers.
2. Background of the Invention
Laser ultrasonic receivers based on optical homodyne interferometers have been investigated for some years. Such receivers have been used and proposed for the examination of materials, such as, for example, investigating transient body transformations, inspecting materials such as metals and ceramics at high temperatures for process and quality control, detecting flaws as soon as they are created, measuring production parameters such as thickness and temperature, and determining microstructural properties on-line such as grain size, porosity and the like. In early research, it was realized that a homodyne interferometer could not operate effectively with the speckled beams that result from reflecting from rough surfaces. Furthermore, such early homodyne interferometers could not compensate for aberrations in the signal beam wavefront resulting from slow, dynamic environmental disturbances.
Time-delay or self-referencing interferometers have been developed, such as the confocal Fabry-Perot which allow the processing of light scattered from rough surfaces with a large field of view. Usually, a phase modulated signal beam is derived from a probe beam scattered or reflected from a vibrating test surface. This beam is demodulated by the slope of the transfer function, which is the transmission versus frequency, of the confocal Fabry-Perot. As a self-referencing or time-delay interferometer, the confocal Fabry-Perot has the ability to process speckled beams from imperfect surfaces. In addition, the particular mirror curvature of the confocal Fabry-Perot provides a much larger field of view than a Fabry-Perot with flat mirrors. The operation of the confocal Fabry-Perot is described in, for example, U.S. Pat. No. 4,659,224. However, the confocal Fabry-Perot requires stabilization of the interferometer length to a fraction of an optical wavelength, thereby adding complexity and cost to the receiver.
The transmitted signal from a confocal Fabry-Perot is proportional to the amplitude of the Doppler shift of the signal beam frequency upon scattering from a vibrating surface. For constant displacement, the Doppler shift decreases with frequency. As a result, the confocal Fabry-Perot does not work well at low ultrasonic frequencies below approximately one megahertz (1 MHz). Solutions to such problems and limitations have been proposed. See, for example, U.S. Pat. No. 5,131,748 to Monchalin and Ing, where the beam that probes the vibrating surface is caused to interfere inside a photorefractive material with a reference or pump beam, resulting in these two beams diffracting in each other's direction with a common path and a common wavefront. An electrical signal dependent on phase excursions or perturbations in the reflected or scattered beam produced by the surface vibration is then obtained by a photodetector in one of these paths. For the correct static phase difference between the wavefronts of the two interfering beams, the electrical signal is linearly proportional to the phase excursion and thus to the surface deflection. The photorefractive material acts in effect as a real-time hologram providing an exact overlap of the reference beam with the signal beam for later coherent detection and it compensates for low frequency dynamic environmental distortions in the signal wavefront. However, most materials used previously do not have both a fast response time and a large diffraction efficiency which is desired for uses in many applications. Such systems also do not operate well at low signal beam light levels produced when scattering from a rough surface, as is typical for many workpieces.
It is still desired to obtain a homodyne interferometer that is characterized by a very fast response time, on the order of microseconds (at intensities on the order of 10-100 milliwatts per square centimeter (mW/cm.sup.2)), while maintaining high diffraction efficiency. It is still desired further to provide a homodyne interferometer that will have the capability of processing speckled returns from the workpiece with a high field-of-view or etendue. It is desired yet further to obtain a homodyne interferometer having an adaptive holographic beamsplitter which can be fabricated more easily, with greater flexibility and is capable of being fabricated (or grown) more rapidly and at lower cost.